These days, a photon counting computed tomography (CT) device is known in which a detector that implements the photon counting technique is used. Unlike an integral-type detector, a detector implementing the photon counting technique outputs signals that enable individual counting of X-ray photons that have passed through a test subject. Hence, in a photon counting CT device, it becomes possible to reconstruct X-ray CT images having a high signal-to-noise ratio (SN ratio).
Besides, the signals output by a detector implementing the photon counting technique can be used in measuring (differentiating) the energy of the X-ray photons. Hence, in a photon counting CT device, imaging can be done by dividing projection data, which is collected by bombarding X-rays of one type of X-ray tube voltage, into a plurality of energy components.
As a detector implementing the photon counting technique, an “indirect-conversion-type detector” is known in which the incident X-ray photons are temporarily converted into a visible light (a scintillator light) using a scintillator and then the scintillator light is converted into electrical signals (an electrical charge) using an optical sensor such as a photomultiplier tube. Herein, the optical sensor individually detects each scintillator photon obtained by conversion of radiation by the scintillator, and then detects the radiation falling on the scintillator and measures the energy of that radiation.
In order to accurately obtain the photon energy of radiation, the quantity of electrical charge that is generated inside the detector due to the incoming radiation photons needs to be analyzed at a high speed and over a wide dynamic range. Conventionally, a method is known in which an integrating circuit, a sample hold circuit, and an analog-to-digital (AD) converter are used. According to that method, with respect to a wave-height pulse that is proportional to the quantity of electrical charge output by the integrating circuit, sampling/holding is performed and is followed by AD conversion using the AD converter.
However, in the conventional technology, a capacitor disposed in the integrating circuit for the purpose of integrating the electrical charge decides on the upper limit of the electrical charge that can be integrated. Hence, it is difficult to achieve a wide dynamic range.
It is an object of the invention to provide a signal processing device and a signal processing method that enable achieving a high count rate and a high resolution.